Within a power meter, a power detector, for example, is used. The measurement value derived from an applied alternating signal—the measurement signal—is obtained either from the heat resulting from absorption or by squaring the signal voltage and subsequent averaging. In general, the measurement signal is an electrical voltage or an electrical current; however, in principle, it could also be any other physical parameter.
In the case of the diode-based power sensors, a squaring of the signal voltage is achieved on the basis of the nonlinear characteristic of the diode, which, in the case of small levels, provides a linear dependence of the output voltage on the input power: see, for example, published patent application DE 199 13 338 A1. On the one hand, the detector element should comprise a good dynamic behaviour which is achieved with a relatively low time constant τ. In this context, the time constant is the product of the zero-point resistance R0 of the diode and the capacitance C of the charging capacitor of the detector element. On the other hand, this detector element should provide a low lower cut-off frequency, in order to detect even very low-frequency input signals. This cut-off frequency is also formed from the zero-point resistance R0 and the capacitance of the charging capacitor. Because of this linking, a small-time constant τ achieves a high cut-off frequency and vice versa. For example, the cut-off frequency of a detector element for time constants τ between 10 nanoseconds up to 1 microsecond is disposed at 10 megahertz to 500 megahertz. With frequencies below the cut-off frequency, an increasingly negative measurement deviation occurs, or respectively, in the case of its correction, a loss of sensitivity, which, in the case of a DC voltage, is 100%. The detector element accordingly has a high-pass behaviour, which makes the detection of electrical powers of low-frequency signals impossible.
In order to make power measurements with signals with a frequency from 9 kilohertz, as is frequently required in the case of EMV applications, a very large time constant τ within the range from 500 microseconds must be taken into account. With such large time constants τ, time variations of the envelope power cannot be measured because of the very low video bandwidth of 300 Hertz. In the case of a measurement of the mean power value, relatively long waiting times must be waited after a relatively large power variation or overdriving of a measurement path. This disadvantageously leads to a lengthening of measurement times.
One object of the invention among other is therefore to provide a power meter which, on the one hand, achieves a very good dynamic performance and can therefore react to strongly or respectively rapidly changing envelope powers of the input signal without long prolongations of measurement time, and, on the other hand, achieves a low lower cut-off frequency in order to detect the power of low-frequency signals.